Designing safer chemicals: Predicting the rates of metabolism of halogenated alkanes

A computational model is presented that can be used as a tool in the design of safer chemicals. This model predicts the rate of hydrogen-atom abstraction by cytochrome P450 enzymes. Excellent correlations between biotransformation rates and the calculated activation energies (ΔH(act)) of the cytochrome P450-mediated hydrogen-atom abstractions were obtained for the in vitro biotransformation of six halogenated alkanes (1-fluoro-1,1,2,2- tetrachloroethane, 1,1-difluoro-1,2,2-trichloroethane, 1,1,1-trifluoro-2,2- dichloroethane, 1,1,1,2-tetrafluoro-2-chloroethane, 1,1,1,2,2- pentafluoroethane, and 2-bromo-2-chloro-1,1,1-trifluoroethane) with both rat and human enzyme preparations: ln(rate, rat liver microsomes) = 44.99 - 1.79(ΔH(act)), r² = 0.86; ln(rate, human CYP2E1) = 46.99 - 1.77(ΔH(act)), r² = 0.97 (rates are in nmol of product per rain per nmol of cytochrome P450 and energies are in kcal/mol). Correlations were also obtained for five inhalation anesthetics (enflurane, sevoflurane, desflurane, methoxyflurane, and isoflurane) for both in vivo and in vitro metabolism by humans: in[F^- ](peak plasma) = 42.87 - 1.57(ΔH(act)), r² = 0.86. To our knowledge, these are the first in vivo human metabolic rates to be quantitatively predicted. Furthermore, this is one of the first examples where computational predictions and in vivo and in vitro data have been shown to agree in any species. The model presented herein provides an archetype for the methodology that may he used in the future design of safer chemicals, particularly hydrochlorofluorocarbons and inhalation anesthetics.